Field of the Invention
This invention relates to reflectors for time-of-flight mass spectrometers, and especially their design.
Description of the Related Art
Instead of the statutory “unified atomic mass unit” (u), this document uses the “dalton” (Da), which was added in the last (eighth) edition of the document “The International System of Units (SI)” of the “Bureau International des Poids et Mesures” in 2006 on an equal footing with the atomic mass unit. As is noted there, this was done primarily in order to allow use of the units kilodalton, millidalton and similar.
In the prior art, there are essentially two types of high-resolution reflector time-of-flight mass spectrometers, which are characterized according to the way the ions are injected.
Time-of-flight mass spectrometers with axial injection include mass spectrometers which operate with ionization by matrix-assisted laser desorption (MALDI). They usually have Mamyrin reflectors (“The mass-reflectron, a new nonmagnetic time-of-flight mass spectrometer with high resolution”, Sov. Phys.-JETP, 1973: 37(1), 45-48) in order to temporally focus ions which have an energy spread. Mamyrin reflectors allow second-order temporal focusing of ions of the same mass but with slightly different kinetic energies. Since point ion sources are used in MALDI ionization, the reflectors can be gridless, as a modification of the Mamyrin reflectors, which are operated with grids in order to limit the fields. MALDI-TOF MS are operated with a delayed acceleration of the ions in the adiabatically expanding laser plasma and with high accelerating voltages of up to 30 kilovolts; in good embodiments, with a total flight path of around 2.5 meters, they achieve mass resolution of R=50,000 in a mass range of around 1000 to 3000 daltons.
Time-of-flight mass spectrometers in which a primary ion beam undergoes pulsed acceleration at right angles to the original direction of flight of the ions are termed OTOF-MS (orthogonal time-of-flight mass spectrometers). FIG. 1 depicts a simplified schematic of such an OTOF-MS. The mass analyzer of the OTOF-MS has a so-called ion pulser (12) at the beginning of the flight path (13), and this ion pulser accelerates a section of the low-energy primary ion beam (11), i.e., a string-shaped ion packet, into the flight path (13), at right angles to the previous direction of the beam. The usual accelerating voltages, only small fractions of which are switched at the pulser, amount to between eight and twenty kilovolts. This process creates a ribbon-shaped secondary ion beam (14), which consists of individual, transverse, string-shaped ion packets. Each of these string-shaped ion packets is comprised of ions of the same mass. The string-shaped ion packets with light ions fly quickly; those with heavier ions fly more slowly. The direction of flight of this ribbon-shaped secondary ion beam (14) is between the previous direction of the primary ion beam and the direction of acceleration at right angles to this, because the ions retain their speed in the original direction of the primary ion beam (11). A time-of-flight mass spectrometer of this type is also usually operated with a Mamyrin energy-focusing reflector (15), which reflects the whole width of the ribbon-shaped secondary ion beam (14) with the string-shaped ion packets, focuses its energy spread, and directs it toward a flat detector (16). The width of the ion beam means the reflector must be operated with grids in order to generate a reflection field which is homogeneous across the width of the ion beam. Mass resolving powers of around R=40,000 at mass 1000 daltons are achieved in these OTOF mass spectrometers.
In a Mamyrin reflector, the ions are decelerated in a homogeneous electric field until they come to a standstill, and are then accelerated again to their original kinetic energy in the reverse direction. The standstill means that the tiniest electric field inhomogeneities have a very major effect on the ions; the generation of the field must therefore be very precise.
Faster ions penetrate slightly deeper into the reflector than slower ions of the same mass; they then obtain slightly more energy on their return journey and catch up with the slower ions precisely at the detector. This is how the velocity focusing works.
It is possible to use a reflector with a single field which is homogeneous throughout. In this case, the length of the reflection field must have a specific, accurately maintained ratio to the total length of the flight path. Since it is often very difficult to fulfill this condition, it is usual to use a shorter, two-part Mamyrin reflector. This comprises a first, relatively strong deceleration field, and then a second, significantly weaker reflection field, in which the ions are brought to a standstill and reflected. This two-part Mamyrin reflector is much easier to adjust electrically, since two voltages are used. In FIG. 1, the deceleration field is generated between the two grids (18) and (19).
As a rule, the Mamyrin reflectors are manufactured from parallel metal plates with large apertures, to which the increasing potentials are applied in the form of voltages. Voltage dividers made from precision resistors are usually used to maintain a potential which increases as uniformly as possible, and thus an electric field which is as homogeneous as possible. The number and spacings of the metal plates and the size of the apertures have been optimized over many years by the manufacturing companies. Thirty to forty of these plates are usually required. The metal plates should be manufactured with precision and also be mechanically strong in order to prevent bending, and particularly vibrations, which can be resonantly generated by rotating pumps and other exciters. In two-stage reflectors, the grids are held by two such plates. FIG. 2 shows part of a reflector which is constructed from simple plates. Insulating spacers (22) ensure the precise separations. The structure is firmly held together by insulating posts (23), which run through the interior of the spacers.
Some commercial time-of-flight mass spectrometers use metal plates whose edge is folded over in an L shape inside the reflector to shield against the ground potential penetrating through from the outside. Part of a reflector with such an arrangement is shown in FIG. 3. The arrangement looks very simple. However, since high mechanical precision is required, these plates with their folded edges are frequently machined from solid material, which means they cannot be manufactured at low cost. The number of plates and voltages can be reduced compared to the reflector in FIG. 2, but between twenty and thirty of these plates are nevertheless required for one reflector. The outer surfaces of the plates are used for the mounting.
Significant progress in reflector technology was achieved by moving the internal shielding edges, which can be seen in FIG. 3, further outwards. FIG. 4 shows that the potential in the interior is now essentially formed by the tabs (27), with the potential of the shielding edges penetrating to only a slight degree. The resolving power of a reflector with this structure is approximately ten to fifteen percent higher than that of a conventional reflector, as shown in FIG. 2 or 3.
In the current state of the art, it remains a challenge to generate a homogeneous deceleration and re-acceleration field in the interior of the reflector. At present, this has to be optimized with a time-consuming voltage adjustment step. There is therefore still a need for a reflector which is simple to manufacture with a high degree of precision and mechanical strength, and which provides an electric field in the interior which is as homogeneous as possible.